Method and apparatus for producing fibre pulp and fuel out of municipal waste

ABSTRACT

Method and apparatus for producing fibre pulp and fuel out of municipal waste or for producing fibre pulp out of waste paper. To control the microbe content, the retention time of the process is kept shorter than 12 hours, advantageously shorter than 6 hours. This is achieved by minimizing the total liquid volume of the process and by increasing the amount of water being passed to water treatment from the process, which water, having been purified, is possibly returned to the process. The method may also include a phase in which the pulp is treated thermally in order to destroy the microbes present in it.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application is a U.S. National Stage application ofInternational Application No. PCT/FI02/00744, filed Sep. 19, 2002, andclaims priority on Finnish Application No. 20011848, Filed Sep. 19,2001.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The invention relates to a method for producing fibre pulp andfuel out of municipal waste. In the method the waste is pre-treated andthe pre-treated waste is pulped in order to fiberize the paper containedin it in water. Two material flows are removed from the pulper, thefirst of which mainly contains fibre suspension and the second mainlycontains indecomposable reject. The fibres contained in the firstmaterial flow are recovered and converted into fibre pulp, and thereject contained in the second material flow is correspondinglyrecovered and converted into fuel. The method according to the inventionalso helps in recovering other waste components, such as metals, fromdry waste in a clean enough state for efficient recycling.

[0004] The invention also relates to a method for producing fibre pulpout of waste paper.

[0005] The invention also relates to apparatus for carrying out theabove-mentioned methods.

[0006] Domestic, industrial and commercial waste flows contain a lot ofusable material the recycling of which would be sensible andenvironmentally friendly if it could be carried out in an economicallyprofitable fashion. Traditional alternatives of waste disposal andprocessing have been transportation of waste to a landfill, incinerationin mass incineration plants and recycling of recyclable materials.Traditional recycling methods require the waste to be carefullyseparated at source and the different waste components to be collectedseparately. Source separation is, however, always to some extentimperfect and due to this the amount of mixed waste is significant nowand will also be significant in the future.

[0007] Lately, there has been an increase in the popularity of wasteenergy recovery by burning quality graded recycled fuel (REF, RDF)converted from waste either in traditional boilers additionally to someother fuel or as main fuel in incineration plants especially designedfor recycled fuels. Compared to a traditional mass incineration plant ofmixed waste this provides lower costs and better efficiency ofelectricity and heat production. Waste best suited for energy recoveryincludes industrial and commercial packaging, paper and plastics wasteand construction waste, which may together make up as much as 70-80% ofthe amount of waste originating from these sources and which amount isusually transported to a landfill. Domestic dry waste can also be usedin energy production as long as e.g. metals, glass and organic wastehave first been separated from it.

[0008] The refining phases of recycled fuel typically include removal ofoversized objects, crushing of waste, separation of metals and removalof sand and stones. The finished recycled fuel contains mainly plastics,wood, paper and textiles. The amount of incombustible impurities in thefuel may be for example of the order of about 5%, depending on thesorting process. Recycled fuel is most advantageously produced out ofthe energy fraction separated at source and collected separately, inwhich case the finished fuel is called REF fuel (recovered fuel).Recycled fuel can also be produced out of unsorted mixed waste by meansof mechanical treatment processes, in which case the finished product iscalled RDF fuel (refuse-derived fuel). The treatment of mixed wasterequires more sorting phases than the treatment of dry waste separatedat source. Heavy material, such as food remains can be efficientlyseparated from the fraction intended for incineration, for example ingravimetric (i.e. based on the size and density of the particles)sorting phases. The composition of finished RDF fuel resembles that ofREF fuel to a great extent, but the proportion of impurities in it maybe a bit bigger than in the REF fuel, for example of the order of about8%.

[0009] Patent publications describe many different methods forrecovering paper fibres and/or combustible material suitable for use asrecycled fuel from waste and particularly from municipal waste. None ofthe known methods has, however, been very popular due to disadvantagescommonly related to them.

[0010] U.S. Pat. Nos. 4,026,678; 4,049,391 and 5,009,672 describemethods for producing recycled fuel out of municipal waste.

[0011] Publication FI 54936 describes a method and an apparatus forrecovering usable material from municipal waste. A mixture of waste andwater is agitated intensively by means of mechanical shearing forces, asa consequence of which the refuse particles become smaller and the papercontained in the refuse is fiberized. Non-fiberized material and fibroussludge, containing, in addition to fibres, a large amount of impurities,are removed from the wet disintegration vessel. Impurities are extractedfrom the fibre-containing sludge by means of centrifugal cleaning andscreening. The rejects from the cleaning phases are passed toincineration, composted or converted into animal feed or into hardboard.The rather similar particle size and specific gravity of paper fibresand of some other components contained in municipal waste has proven tobe problematic in view of fibre recovery. To ensure a good enoughquality in the fibre pulp recovered the amount of reject must be kept ata high level in the centrifugal cleaning and screening phases. As aconsequence, over 25-60% of the fibre originally fed into the system hasto be rejected.

[0012] Publication U.S. Pat. No. 5,100,066 describes a method with whiche.g. fuel and paper fibre are produced out of municipal waste. The wasteis pre-treated and a light fraction containing paper, textiles andplastics is separated from it, which light fraction is washed and pulpedin order to fiberize the paper contained in it in water. The sludgeproduced in pulping is screened to separate the fibres from theindecomposable reject, which is passed into fuel production. The fibresuspension is pressed dry and the paper fibres are delivered to thepaper mill. Waste waters from screening and fibre pulp pressing arepurified and returned to the pulper. In this process fibre recovery alsoremains poor, since fibres are lost both at the washer preceding thepulper and on the screen following pulping, from which screen some ofthe fibres are removed with the reject flow to incineration. The qualityof the recovered fibre pulp is poor, because it is not sorted in any wayafter the screening following pulping and before pressing and deliveryto the paper mill. Also, the quality of the fuel produced in the processis poor, as it is composed of mixed refuse material.

[0013] A problem associated with the recovery of municipal waste is thelarge microbe content of the waste, which content still tends to growduring the processing of the waste. This may lead to problems in thedifferent phases of the process and the problems may also haverepercussions in the fibre pulp produced out of the waste. A high levelof microbial activity may also cause problems with e.g. hygiene andodors as well as lead to fibre loss and poor quality of fibre. Someknown arrangements have tried to avoid problems caused by microbes byseparating fibres from other waste components in a dry process frombeginning to end. Wet processing has, however, its advantages, such asthe purity and good quality of the fibre pulp and fuel produced.

SUMMARY OF THE INVENTION

[0014] An object of the invention is to reduce problems related to therecovery and recycling of the recyclable fractions of municipal waste.

[0015] An object is especially to provide a process by means of which itis possible to simultaneously produce, out of waste, microbiologicallypure, good-quality fibre pulp and homogenous fuel with a good heatingvalue.

[0016] A further objective of the invention is to provide a process forproducing fibre pulp which meets the quality requirements set for fibreproducts that come into contact with foods, such as corrugated cardboardcases meant for packing fruit and vegetables.

[0017] For the sake of the purity of the fibre pulp it is essential thatthe retention time in the process is kept so short as to prevent themicrobes from reproducing to a detrimental extent. In the methodaccording to the invention the process tanks are designed so that theretention time of the process is less than 12 hours, preferably lessthan 6 hours and most preferably less than 2 hours. The purity of theprocess can also be enhanced by increasing the amount of water beingpassed to water treatment from the process. In such a case filtrates andcontaminated waters of the process are circulated back into use throughbiological treatment, whereby the water circulating through thebiological treatment rinses the process and washes nutrients andmicrobes out of it. In the method according to the invention the amountof water circulated through the biological treatment is 5-50 m³,advantageously 10-30 m³, per fibre ton recovered.

[0018] The reproduction of microbes in their growth phase can beillustrated simply with the following equation:

dC/dt=r C   (1)

[0019] which leads to the following:

C_(t)=C_(o) e^(r t)   (2)

[0020] In these equations:

[0021] dC/dt=increase in the amount of microbes in a time unit,

[0022] t=retention time in the process,

[0023] C_(o)=amount of microbes at the starting point,

[0024] C=amount of microbes in the process at the point of observation,

[0025] C_(t)=amount of microbes in the process after the lapse of timet,

[0026] r=reaction rate constant (growth rate constant) dependent on theconditions.

[0027] For the reaction rate constant r the following equation applies:

r=1/k   (3)

[0028] where k=average division time of microbes in the conditions inquestion.

[0029] The division time of most microbes that can be found in theprocesses of the paper industry is between a half an hour and a coupleof hours depending, for example, on the acidity and temperature of theprocess as well as on the nutrients contained by the raw material. Inpractice, using biocides can retard the reproduction of microbes and theproblems caused by them.

[0030] The exponential growth of the microbe content can be illustratedwith an example. If the number of microbes contained in the raw materialis C_(o), the division time of the microbes being one hour, after twohours the number of microbes is C_(o) ² and after six hours C_(o) ⁶.Correspondingly, after 24 hours the number is C_(o) ²⁴, provided thatthe nutrients present in the waters do not constrain the bacterialgrowth.

[0031] In the existing recycled fibre processes the retention times are,in practice, more than 24 hours, even several days. The retention in theprocess and intermediate tanks of the pulp department can be severalhours. In the pulp tower, for example, it is often 8-12 hours, and thewater tower of the pulp department is usually of a similar size and theclear filtrate tower likewise. For the sake of the controllability ofthe process shorter retention times have not really been accepted intraditional processes. In order to be able to control the heavy slimebuild-up and paper machine problems caused by microbial growth, thewater circulation of the machines is dosed with biocides againstmicrobes, with which the amount of bacteria is in most cases constrainedto a level of 10⁵-10⁷ colonies/ml.

[0032] It can be concluded from the simple calculation presented abovethat by reducing the retention time of the process from 24 hours to 6hours the amount of bacteria produced by growth can be decreased at bestby an order of magnitude of C_(o) ¹⁸ meaning that the bacteria contentof the product is reduced to a billionth of a billionth. In practice,the difference is not quite as significant due to the use of biocidesand a division time which is an hour longer, on average. It is clear,however, that by means of the process according to the invention, inwhich process the retention time is only a few hours, both the need touse biocides and the amount of bacteria carried with the product floware very decisively reduced compared to the traditional process, inwhich the retention time is long.

[0033] A short retention time can be brought about by keeping the totalliquid volume of the process small enough. This objective can beillustrated by means of an equation depicting the retention time t:

t=V _(p)/(F _(K) +F _(R) +F _(B))   (4)

[0034] where

[0035] V_(p)=total liquid volume of the process equipment,

[0036] F_(K)=volume flow discharging together with the fibres,

[0037] F_(R)=volume flow discharging together with the reject, and

[0038] F_(B)=volume flow passed to a biological treatment plant.

[0039] The equation illustrates well the average retention time ofwaters within the balance boundaries of the factory. The real retentiontime differs from this slightly since the water exchange rate varies indifferent tanks.

[0040] Said total liquid volume of the process equipment comprises theliquid volume of all pulp, water and reject tanks, the liquid volumeincluding pulps and waters. If, for example, the reject and pulp tanksare full, the water reservoir is typically empty and vice versa. What ismeant is the volume in real process usage and not the calculated volumeof all the tanks added together. This is an essential piece ofinformation in view of the reproduction of the bacteria suspended in theliquid phase. With respect to bacterial growth it is also essentialthat, when designing the tanks and piping, blind angles, in which themicrobes are able to reproduce freely, be avoided.

[0041] The retention time can be shortened, for example by minimizingthe water volumes of the process and by recycling most of the watercontaminated in the process back into use through the biologicaltreatment.

[0042] In traditional arrangements attempts have been made to minimizethe amount of water passed to waste water treatment, and in most simplerecycled paper mills this amount is 3-5 m³/t. In cases where the watercirculation has been closed by connecting the biological treatment toit, the amount of water circulating through it is of this order ofmagnitude.

[0043] In the method according to the invention the amount of water isnot minimized, but instead, the biological treatment is part of thepurity control of the process. A sufficient amount of water is takeninto the biological treatment from the most contaminated phases of theprocess, and circulating the water through it reduces the growth ofmicrobes in the process. The majority of the microbes coming into thebiological treatment with the water are removed in the treatment and inthe separation/clarification equipment, which is an essential part ofit. The water returning to the process from the biological treatment canalso be sterilized by using methods known as such, like UV-light,chlorination or ozonization. By means of sterilization biologicalactivity can be done away with either completely or, if needed, onlypartly. Thus, the water circulation running through the biologicaltreatment plant rinses and cleans the whole recovery process bydelivering biological material into a cleaning plant, where it isremoved from the water circulation, and thus reduces the biologicalactivity in the fibre pulp produced. In order to achieve a good endresult at least 5 m³ and not more than 50 m³, advantageously 10-30 m³,of water per fibre ton produced by the recovery plant should be passedto the biological treatment. Using more water than this in thecirculation does not essentially decrease the level of the biologicaloxygen demand (BOD) of the process water. Instead, excessive increasingof the circulation adds losses.

[0044] The growth rate of microbes depends, among other things, on thetemperature and pH and on the amount of nutrients, such as phosphor,nitrogen and BOD. The amount of nutrients in the process is reduced whena major part of the water used in the process is circulated throughbiological treatment. Thanks to efficient water treatment and shortretention times the pH of pulping rises to over 7.5, whereas it normallyis below 7. The biological treatment efficiently removes organic acidsfrom the waters by breaking them down into carbon dioxide and oxygen. Atthe same time some of the acids is replaced by bicarbonate ions. As aconsequence, the pH of water settles between 7.0-8.5 without usingchemicals. A high pH also makes the calcium carbonate, present in thepaper as filler material, dissolve to a lesser extent, thereby improvingyield and reducing the amount of waste.

[0045] The purity and hygiene level of the pulp produced out of wastefibres can be further enhanced by dispersing, whereby the pulp is heatedto a temperature of about 100° C., up to 130° C., if necessary, and isdefibrated at a consistency of about 30-40%, whereby the stickiespresent in the pulp are broken down. The retention time of pulp indispersing is usually only 5-20 seconds. By increasing the retentiontime of pulp at a high temperature and consistency it is possible tofurther weaken the living conditions of microbes, whereby pure andsterile pulp is produced. In the method according to the inventiondispersing is followed by hot storage, the duration of which may be2-120 minutes. The minimum duration of hot storage is selected,depending on the temperature, so that the sum of temperature determinedby the foodstuffs legislation for the sterilization of foods is reached.

[0046] A similar thermal treatment of pulp can also be carried out inconnection with the drying of product pulp to be baled. The drying canbe carried out by means of hot air, flue gas or in a steam atmosphere.The steam dryer can be a construction that passes the pulp pneumaticallythrough heat exchangers, in which construction the retention time istypically 0.2-5 minutes, or a fluidised-bed dryer or a drum dryer, inwhich the retention time in superheated steam may be several tens ofminutes.

[0047] Alternatively, the sterilization of pulp can also be carried outby means of bleaching by selecting such bleaching conditions, chemicalsand retentions that the product is sterilized.

[0048] The ash content of fibre pulp produced out of waste fibres can beinfluenced by adjusting the recovery degree of ash in connection withthe washing of the pulp. A washer suited for controlling the ash levelis, for example, a GapWasher™ manufactured by Metso Paper, Inc. Thefiltrate from the washer is passed to microflotation or microfiltration,from where the ash is taken out of the process together with the sludge.The ash can also be partly or completely returned to the processdepending on the properties of the raw material and the requirements ofthe product.

[0049] The use of recycled products made of waste fibres in thepackaging of food products is limited by strict purity requirements. Bymeans of the operations presented above it is possible to achieve, inthe end product, a purity level at which board made of recycled fibremeets the requirements set for products used in connection with foods.

[0050] When pulping pre-treated municipal waste two material flows areseparated from each other, the first of which is mainly composed of thewater suspension of fibres and the second mainly of reject which was notdecomposed during pulping. The separation into two material flows takingplace in connection with the pulper is, of course, imperfect. Indeed, anadditional characteristic of the invention is the recovery of fibresfrom the reject-containing material flow. This is carried out by washingthe reject with circulation water from which the fibres have beenrecovered, and by passing part of the fibre-containing wash waters andthe fibres recovered from the circulation water to be mixed with thefibre-containing material flow. In this way it is not only possible toincrease the fibre recovery of the process, but also to enhance theheating value of the fuel produced out of the reject, since the heatingvalue of fibres is, as is well known, significantly lower than that of,for example, plastic, which makes up the majority of the reject leavingthe pulper.

[0051] The quality and purity of the fuel produced out of reject canalso be enhanced by removing PVC plastics from it e.g. by using nearinfrared technology and the aluminium foils, for example, by means ofeddy-current technology. The result is a fuel the chlorine and aluminiumcontent of which are so low that it can safely be burned in traditionalboilers. The sodium chloride contained in the waste is dissolved inwater in connection with pulping, which helps in achieving low chlorinecontent. The heating value of the plastic-containing fuel produced withthe method according to the invention is 20-40 MJ/kg, whereas theheating value of, for example, fuel produced out of domestic waste isless than 10 MJ/kg and the heating value of pure wood is 17-18 MJ/kg.

[0052] The invention will now be described in more detail with referenceto the figures of the accompanying drawings, to the details of which theinvention is, however, by no means intended to be narrowly confined.

BRIEF DESCRIPTION OF THE DRAWINGS

[0053]FIG. 1 shows fractionation of waste into different fractions inthe various phases of the treatment.

[0054]FIG. 2 shows pulping of waste and the following process phases inthe manufacture of fibre pulp and fuel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055]FIG. 1 schematically shows a process for producing fibre pulp andfuel, in which industrial, commercial or domestic energy waste separatedat source and mainly composed of wood, plastics, paper and board andother combustible substances, is used as a raw material. Mixed waste mayalternatively be used as a raw material, in which case the removal oforganic waste requires more effort.

[0056] The waste material is passed from an intermediate storage 40 tocrushing devices 41, where it is broken down into pieces of the sizesuited for further treatment. After the crushers 41 ferrous metals areremoved from the waste proceeding on the belt by means of a magneticseparator 42. The waste material is passed onto a star screen 43, wherea light fraction containing paper and plastics is separated from it bymeans of air flow (suction or blowing). A fraction containing large andheavy particles is left on the screen 43, which fraction contains, forexample, wood, heavy plastics and metals. Fines and organic wastedischarge through the slots of the screen 43.

[0057] The light fraction separated from the waste with the star screenis passed onto a wind sieve 44, where objects with heavy specific weightare separated from it by means of air flow and gravity, which objectsare passed to be mixed with the heavy waste fraction leaving the starscreen 43. The pure light fraction is passed to a pulping station 45.

[0058] At the pulping station 45 the waste is diluted with water and itis agitated intensively, whereby the paper and board present in it arefiberized in water. Debris particles are removed from thefibre-containing sludge and it is passed to fibre processing 47. Reject,containing among other things plastics and aluminium foils, which wasnot decomposed during pulping, is passed to reject processing 48, wherealuminium is separated from it with an eddy-current separator and PVCplastics with a near infrared separator. These separation phases producea plastics fraction rich in polyethylene and polypropylene plastics,which fraction is well suited to be used as fuel or as a raw materialfor plastic oil. The aluminium can be reused and the PVC plastics can beused as mixed fuel in energy production, whereby chlorine emissions atthe cleaning devices following the boiler are prevented.

[0059] At the pulping station 45 fibre-containing rejects are produced,which can be used, e.g. as fuel. The filtrates and contaminated watersof the pulping station 45 are passed to purifying equipment 51—the“kidney” of the process—and from there further back to the process. Thekidney includes a biological treatment plant and the needed number ofother water purifiers.

[0060] The heavy waste fraction from the star screen 43 is passed to ametal separation 46, where metals are separated from it, for example bymeans of a magnetic separator, an eddy-current separator based on theinductivity of metals and/or a vibrating screen. After the removal ofmetals the waste is passed to a water bath separator 52, where heavysubstances, such as glass and stones, but also fine substances likesludge and organic waste are separated from it by means ofclarification. From the water bath separator 52 the waste is passed to asecond wind sieve 53 of the process, where, by means of an air flow,wood is separated from other waste, which at this point comprises mainlyplastics. Plastics waste can be recovered as such or it can be passed tothe separation phases 48 of aluminium and PVC together with theindecomposable reject from pulping 45.

[0061] Contaminated wood containing wood preservatives or othercontaminants can further be separated from the wood fraction recoveredfrom the wind sieve 53 by means of an X-ray device 54 or another knownseparation method. After this, the contaminated wood can be incinerated,for example in a fluidised-bed boiler having flue gas purificationequipment required by the EU Waste Incineration Directive. Pure wood canbe used as a raw material for chipboard or as fuel.

[0062] From the process shown in FIG. 1 a pure fibre fraction anddifferent wood, plastics and metal fractions are recovered, which may bereused either as a raw material or as fuel. A further alternative is toprovide, between the wind sieve 44 and the pulping station 45, equipmentfor separating different paper fractions from each other on the basis ofcolor, whereby brown and white paper can be passed to different pulpinglines and two different pulps can be produced out of the waste.

[0063] All the waste pre-treatment phases before pulping 45 and thewater bath separator 52 are carried out as a dry process without addedwater. Due to the specific quality of the raw material the devices usedin crushing and fractioning tend to get dirty, which means that theiroperating efficiency is diminished. According to a furthercharacteristic of the invention certain process devices, such as thecrushers 41 and the star screen 43 are equipped with wash water sprays,with which said process devices can be cleaned from the dirtperiodically or whenever necessary. Circulation water, for examplemechanically purified water from the water bath separator 52, is usedfor washing. The wash water may also be used for decreasing the firerisk and for preventing dusting. The possibility of using circulationwater in the pre-treatment process in the way illustrated abovesignificantly improves the reliability and occupational safety of thewaste treatment plant.

[0064]FIG. 2 shows in more detail the process phases associated with thepulping of pre-treated waste and with the recovery of fibres and rejectat the pulping station 45 of FIG. 1 as well as the purificationcirculation of the waters used in the recovery process.

[0065] In a pulper 11 pre-treated dry waste 10 is diluted by means ofcirculation water into a mixture, the consistency of which is about5-20%. The mixture is agitated intensively, whereby the paper and boardmaterial present in the waste is fiberized in water. The fibresuspension is passed out of the pulper 11 through a fixed screen platein the pulper, the diameter of the holes of said screen plateadvantageously being about 5-20 mm. A second material flow containingfibres and water, but also material that is not decomposed in pulpingi.e. reject (e.g. plastics, wood and textiles) is removed from thepulper 11 by means of a feed screw 12. Large, heavy particles (stones,sand, glass, etc.) sink to the lowest part of the pulper, from wherethey are removed by means of a separate debris trap (not shown).

[0066] The fibre suspension is first passed onto a coarse screen 20,which is advantageously a rotary drum screen, the diameter of whoseholes is advantageously in a range of 1.6-3.0 mm. The pulper mayalternatively be equipped with a rotor unit according to FI patent82493, in which unit coarse separation of the pulp is carried out at thesame time. On the coarse screen 20, e.g. plastics, slivers and other bigcontaminants are removed from the fibre suspension and are passed to bemixed with the reject flow discharging from the pulper 11. The acceptfrom the coarse screening is passed to hydrocyclones 21 and further tofine separation 22. The reject from the fine separation 22 is passedthrough a press 29 and is thus converted into fuel. The separated pulpis washed with a washer 23, which is advantageously a GapWasher™ washer.With this washer 23 inorganic material (ash) can be washed out of thepulp, which inorganic material is further removed from the filtrate ofthe washer by means of microflotation. In addition, the washer thickensthe pulp to a consistency of 8-12%. Part of the wash filtrate isrecycled through fibre recovery 33 to serve as dilution water in theprocess phases before washing and part is passed to be purified in abiological treatment plant 30.

[0067] After the washer 23 the pulp is thickened by means of a screwpress 24 to a dry solids content of about 30-40% and passed todispersing 25. The dispersing is carried out at a temperature of about100-130° C. After the dispersing 25 the pulp is transported into astorage tank 26, where the temperature and consistency of the pulp aremaintained substantially on the same level as in the dispersing 25. Theduration of hot storage is, depending on the temperature, at least 2minutes and not more than 120 minutes. Prolonged storage at a raisedtemperature and at a high consistency sterilizes the product and ensuresthat the microbiological requirements for the product are met.

[0068] Indecomposable material i.e. reject, mainly comprising fibres,plastics, wood, textile and other relatively light material, iscontinuously removed from the pulper 11 with the screw conveyor 12. Thereject discharging from the pulper 11 is washed with wash water spraysas it moves along the screw conveyor 12 and the washing is continued ina reject washing drum 13. The fibre-containing water gathered in thewater reservoir of the washing drum 13 is circulated through a purifyingdevice 17 back into use. The purifying device 17 is equipped with twoscreening surfaces, of which the first one in the flow direction ismeant for coarse screening (separation of debris particles and slivers)and the second one for fibre recovery. The water cleaned from fibreswith the purifying device 17 is returned back to the process, where itcan be passed, for example, to serve as wash water in the washing spraysof the screw conveyor 12 and into the pulper 11 or to serve as dilutionwater in coarse screening 20. The fibres recovered with the purifyingdevice 17 and the fibre-containing wash water coming from the screwconveyor 12 are passed to be mixed with the fibre suspension dischargingfrom the pulper 11.

[0069] After the washing drum 13 water is removed from the reject bypressing it as dry as possible, to a dry solids content of about 50-70%,by means of a screw press 28. After this, the reject is passed tofurther treatment, where it is converted into fuel or into a form suitedfor recycling the material. The further treatment phases may include,for example, crushing or grinding of the reject and furtherfractionation for producing various fuel or recyclable fractions. Thewaste fuel produced in the way illustrated above is of a higher purityand quality compared to normal recycled fuel produced in a dry process,because organic material with a low heating value and incombustibleinorganic material have been removed from it in connection with the wettreatment.

[0070] 5-50 m³, advantageously 10-30 m³, per fibre ton produced fromfiltrates and other dirty waters generated in the processing of wasteare passed through the fibre recovery 33 to be purified in thebiological treatment plant 30. Before the biological treatment fibres,fines and ash can be recovered, for example by means of a screen and byflotation. By combining the techniques it is possible to adjust thequality of the product to a uniform level to meet the requirements. Thiscan be influenced by returning the fibres from the screen back to theprocess and by taking part or all of the ash-solids mixture separated inmicroflotation back into production. In microflotation it is possible toaffect the separation ratios between ash and other fines by changing therunning parameters. After the biological treatment 30 the purified wateris returned to the process and the excess is passed to a wastewatertreatment plant. If particularly pure water is needed, part of thebiologically treated water can still be treated with ultra- and/ornanofiltration. The water or part of the water to be returned to theprocess can also be sterilized, and in this way living biomass can beprevented from returning to the fibre recovery line.

[0071] Thanks to the biological treatment of circulation water thebiological oxygen demand (BOD) of the process can be brought to the samelevel as in a traditional open water circulation system. As aconsequence the pH of the process rises to a range of 7.5-8.5 and thebiological activity in the water circulation diminishes. The activity ofanaerobic bacteria is essentially reduced and odor nuisances alsodiminish.

[0072] An object of the invention is that the waste material passesrapidly through the treatment process, meaning that there is not enoughtime for the microbes to reproduce significantly during the process. Theexponential effect of the retention time t on the growth of microbesC/dt is illustrated by the equation presented above (2):

C^(t)=C_(o) e^(r t)

[0073] To decrease the retention time the liquid volume V_(P) of theprocess is minimized and the amount of water F_(B) passed from theprocess to water treatment is increased beyond the traditional amount.The retention time t is defined by the equation presented above (4):

t=V _(P)/(F _(K) +F _(R) +F _(B))

[0074] The liquid volume VP of the process means the amount of liquidcirculating in the liquid circulation, outlined in FIG. 2 with dashedlines, between the pulper 11, fibre press 24 and reject press 28. In theexample case the total volume is considered to include the pulper 11,the devices 20, 21, 22, 23, 24, 29 and 33 used in fibre recovery, thedevices 12, 13, 17 and 28 used in reject recovery and the pipingsconnecting them. F_(K) is the volume flow discharging from the processequipment together with the fibres—in FIG. 2 being the water flowleaving the press 24 with the pulp. F_(R) is the volume flow dischargingfrom the process equipment together with the reject—in FIG. 2 being thewater flow leaving the press 28 with the reject. F_(B) is the volumeflow discharging from the process equipment to enter the biologicaltreatment plant, including, in FIG. 2, the filtrates of the pulp washer23 and the press 24 as well as the filtrates of the reject presses 28and 29.

[0075] Not only does the invention help to minimize the retention time tof the process, but it also prevents the minimizing of the process flowF_(B) passed to biological treatment, something that has been commonpractice in the traditional process. In the traditional process thevolume flows F_(K)+F_(R) discharging together with the fibres and thereject are altogether about 1-2 m³/t and F_(B) is typically 3-5 m³/t. Inthe method according to the invention the flow FB is increased to alevel of 5-50 m³/t, advantageously 10-30 m³/t. The flow increase fromlevel 4 m³/t to level 10 m³/t affects the growth of microbes in the sameway as halving the liquid volume of the process according to theequation t=V_(P)/(F_(K)+F_(R)+F_(B)).

[0076] Thanks to the biological treatment of the circulation water thepH of the process is higher than in a corresponding process using freshwater. The circulation of waters through the biological treatmentremoves nutrients from the process waters, whereby the reaction rateconstant r in the equation (2) decreases and the growth of microbesslows down.

[0077] When the fibre pulp is, in addition, treated thermally at araised temperature, the finished fibre pulp does not contain livingbiological material.

[0078] The basic idea of the invention for diminishing microbialactivity by decreasing the retention time of the process can well beapplied also in traditional pulp and paper manufacture using wastepaper. In such a case the amount of reject and the volume flow F_(R)discharging with it are naturally smaller than when using municipalwaste.

[0079] Instead of biological treatment another known water treatmentmethod for reducing the amount of microbes in the water circulation maybe used. Alternatively, the amount of water to be passed to the watertreatment can be substituted with fresh water, when plenty of it isavailable.

[0080] The claims will now be presented, and, within the inventive ideadefined by the claims, the details of the invention may vary and differfrom what is presented above as exemplary only.

1-16. (canceled)
 17. A method for producing fibre pulp and fuel out ofmunicipal waste, comprising the steps of: pre-treating a flow ofmunicipal waste in a dry state; following the pre-treating, treating theflow of municipal waste in a wet process, having a defined liquidvolume, wherein the following process steps occur within the wetprocess: pulping in a pulper the flow of waste with water to fiberizepaper contained within the waste; removing from the pulper a firstmaterial flow containing mainly fiber suspension and removing from thepulper a second material flow containing mainly indecomposable rejects;recovering fibers present in the first material flow and converting therecovered fibers into fiber pulp; recovering the rejects contained inthe second material flow and converting said rejects into fuel; flowinga first water flow from the wet process volume at a first rate tobiological treatment; flowing a second water flow from the wet processvolume at a second rate leaving the wet process volume with the fiberpulp; and flowing a third water flow from the wet process volume at athird rate leaving the wet process volume with the indecomposablerejects, wherein a retention time of the wet process liquid volume isdefined as the wet process liquid volume divided by a sum of the firstflow rate, the second flow rate, and the third flow rate, and whereinthe retention time is shorter than 12 hours.
 18. The method of claim 17wherein the retention time is shorter than 6 hours.
 19. The method ofclaim 17 further comprising the step of reducing the microbe content ofthe rejects and the fiber by washing the rejects and the fiber with thefirst flow of water after it has passed through biological treatment andbeen purified.
 20. The method of claim 19 wherein the the first waterflow rate is 5-50 cubic meters per ton of fiber recovered from the flowof municipal waste.
 21. The method of claim 19 wherein the the firstwater flow rate is 6-30 cubic meters per ton of fiber recovered from theflow of municipal waste.
 22. The method of claim 19 further comprisingperforming the following steps on the first flow of water before thefirst flow of water is used for washing: removing the solids from thefirst flow of water; and sterilizing the first flow of water.
 23. Themethod of claim 17, wherein when processing the fibers the fiber pulp iswashed and the ash content of the pulp is controlled and adjusted inconnection with the washing.
 24. The method of claim 17, furthercomprising dispersing the fibers of the fiber pulp at a temperature of100-130° C. and a consistency of 30-40%, and then holding the fiber pulpat 100-130° C. and consistency of 30-40%, for at least for 2 minutes anduntil norms set for sterilizing of foods are met.
 25. The method ofclaim 17, further comprising bleaching the fibers of the fiber pulpusing such process conditions that microbes present in the fiber pulpare destroyed.
 26. The method of claim 17, further comprising drying thefibers out of the fiber pulp in such a way that microbes present in thefiber pulp are destroyed.
 27. The method of claim 17, wherein fibers arerecovered from the second material flow by washing it with water fromthe first water flow after the first water flow has passed throughbiological treatment and been purified, said fibers recovered from thesecond material flow being passed to the first material flow.
 28. Themethod of claim 17, wherein when processing the second flow containingmainly indecomposable rejects, PVC plastics or aluminum are removed fromit.
 29. The method of claim 17, wherein the pre-treatment of wastebefore pulping comprises: crushing the waste within a waste crushingdevice; removing heavy material in a heavy material removing device; andwashing the waste crushing device and the heavy material removing deviceby passing wash water therethrough.
 30. The method of claim 29, whereinthe wash water is taken from the first water flow after it has passedthrough biological treatment and been purified.
 31. A method forproducing fibre pulp and fuel out of municipal waste, comprisingtreating a flow of municipal waste in a wet process having a definedliquid volume, wherein the following process steps occur within the wetprocess: pulping the flow of waste with water to fiberize papercontained within the waste; removing from the pulper a first flowcontaining mainly fibre suspension and removing from the pulper a secondflow containing mainly rejects; recovering fibers present in the firstflow and converting the fibers into fiber pulp; and from the wet processvolume, flowing a first water flow at a first rate to biologicaltreatment, a second water flow at a second rate leaving the wet processvolume with the fiber pulp, and a third water flow at a third rateleaving the wet process volume with the rejects, wherein a retentiontime of the wet process liquid volume is defined as the wet processliquid volume divided by a sum of the first flow rate, the second flowrate, and the third flow rate, wherein the retention time is shorterthan 12 hours.
 32. The method of claim 31 wherein the retention time isshorter than 6 hours.
 33. The method of claim 31 wherein the rejects andfiber are reduced in microbe content by washing with the third flow ofwater after it has is passed through biological treatment and beenpurified.
 34. The method of claim 33 wherein the the first water flowrate is 5-50 cubic meters per ton of fiber recovered from the flow ofmunicipal waste.
 35. The method of claim 33 wherein the the first waterflow rate is 6-30 cubic meters per ton of fiber recovered from the flowof municipal waste.
 36. The method of claim 33 wherein the solids areremoved from the third flow of water and it is sterilized before it isused for washing.
 37. The method of claim 31 wherein the when processingthe fibers the fiber pulp is washed and the ash content of the pulp iscontrolled and adjusted in connection with the washing.
 38. The methodof claim 31 further comprising dispersing the fibers of the fiber pulpat a temperature of 100-130° C. and a consistency of 30-40%, and thenholding the fiber pulp at 100-130° C. and consistency of 30-40%, for atleast for 2 minutes and until norms set for sterilizing of foods aremet.
 39. The method of claim 31, further comprising bleaching the fibersof the fiber pulp in the bleaching using such process conditions thatmicrobes present in the fiber pulp are destroyed.
 40. The method ofclaim 31, further comprising drying the fibers out of the fiber pulp insuch a way that microbes present in the fiber pulp are destroyed. 41.The method of claim 31, wherein fibers are recovered from the secondflow by washing it with water from the first water flow after it has ispassed through biological treatment and been purified, said fiber beingpassed to the first flow containing mainly fibre suspension.
 42. Anapparatus for producing fiber pulp and fuel out of municipal waste,comprising: devices for pre-treating the waste in a dry state, followedby wet system defining a total liquid volume, the wet system furthercomprising: devices for pulping the waste; devices for recoveringfibers; devices for converting said fibers into fiber pulp; devices forrecovering the reject from pulping and converting it into fuel; meansfor moving total liquid volume through the wet system in less than 12hours.
 43. The apparatus of claim 43 wherein the means for moving totalliquid volume through the wet system in less than 12 hours moves thetotal liquid volume in less than 6 hours.
 44. An apparatus for producingfiber pulp and fuel out of municipal waste, comprising: a wet systemdefining a total liquid volume, the wet system further comprising:devices for pulping the waste; devices for recovering fibers; devicesfor converting said fibers into fiber pulp; devices for recovering thereject from pulping and converting it into fuel; and means for movingtotal liquid volume through the wet system in less than 12 hours. 45.The apparatus of claim 45 wherein the means for moving total liquidvolume through the wet system in less than 12 hours moves the totalliquid volume in less than 6 hours.